In the past years, the use of ceramic powders encapsulated in nanoparticle coatings has been the target of intense worldwide research, for a vast set of applications, in chemistry, biomedicine, electronics, ceramic and energy fields, aiming to benefit from the conjugation of the base particle properties and the unique effects introduced by the nanoparticle layer, which compose the coating of its surface.
Lately, considering the lack of knowledge concerning the impact resulting from nanoparticle absorption in living beings, as well as from the difficulties associated to its handling, the technique related to the adhesion of the nanoparticle layer to the surface of bigger-dimensioned particles has been used in the pursuit for conjugating the unique properties provided by nano dimension, but without the risks associates thereof.
The methods usually used to prepare ceramic coatings, are generally divided into four categories:
I—Coatings Via Chemistry Processes (Wet)
The micro emulsion and sol-gel deposition techniques are examples in this process. In the later, the four main steps are the following:    a) The colloidal particles intended to coat form a stable dispersion in a liquid, which also comprises the coating precursors;    b) These precursor deposition onto the particle surfaces by spraying, immersion or rotation;    c) The particles are polymerized during the removal of stabilizers, thus producing a gel in a continuous net;    d) The final thermal handling results in the pyrolysis removal of the organic materials, leaving a crystalline or amorphous coating.
The main difficulty in coating processes via wet chemistry consists of controlling de reaction speed of the coating particle formation, making it difficult to obtain uniform coatings, with high adhesions.
II—Coatings Via Gaseous Phase Deposition
The technology of chemical vapor deposition (CVD) is a very common example of this category. In this case, the coating precursor is heated and vaporized at high temperature (700-1000° C.), being later deposited in a fixed substrate.
This is a commonly-used method in coating tools; the coating thickness is high, typically between 5 and 12 microns, and there is no individual particle coating, but rather a substrate that might have several geometrical forms.
Another technique is the atomic layer deposition (ALD) that allows forming fine layers in a solid substrate, comprising two steps:    a) A gas, usually a gaseous organometallic precursor, is absorbed at the substrate surface;    b) Reaction of the first gaseous precursor with a second gas, forming a monolayer, the number of reaction cycles between both precursors being the factor that controls the film's final thickness.
Another more elaborated technique consists of the synthesis of the base particles and of the coating particles from two gaseous precursors injected at different times in a hot-wall aerosol flow reactor.
A key example of this technique is the coating of titania particles (TiO2) with silica (SiO2). The major disadvantage of this method consists of the low production volumes and their high costs.
III—Electrochemical Coatings
This approach starts from a suspension of particles intended to coat, to which a cation set is added, the cations being afterwards electrochemically reduced, forming a set of nanoparticles set which are deposited at the surface of the base particle. The formation and deposition of copper oxide (CuO) nanoparticles onto silica particles (SiO2) is a classical example of the implementation of this technique.
IV—Dry Coatings
As an example, the projection technique, wherein a dense ceramic-target comprising the coating material, is sputtered by electrons, thus depositing it almost atom by atom in a substrate, thus forming a film. However, when one tries to coat particles of submicrometric dimensions, it becomes difficult to obtain a homogeneous coating in the particle, notwithstanding the fact that it is a quite costly method.
It is ascertained that the four families of methods previously enumerated present the following limitations:    a. Difficulty in coating individual ceramic particles, being more appropriate for substrate or surfaces coatings. This difficulty increases as the base particle dimension reduces;    b. Extreme difficulty in obtaining homogeneous coatings, essentially in terms of thickness and coated area;    c. Strong limitation on the type of nanoparticle crystalline structures which constitute the coating;    d. Difficulty in producing coatings with excellent adhesion to the base particle;    e. Difficulty in obtaining coatings formed by individualized nanoparticles on the surface of the base particle; normally, the particles are produced with a coat of a continuous film. This is an important limitation, once it doesn't allow benefiting from the advantages of the effects resulting from the nanometric dimension;    f. Extreme high dimension of the base particle crystallite and coating particle due to the thermal treatment step.
In its turn, the method proposed by the present invention consisting of the detonation of an (W/O) emulsion, to which at least a solid precursor was previously added, which decomposes during the emulsion detonation, forming the nanoparticles comprising the desired composition, quantity and crystalline structure for the coating. This method presents an enormous versatility, given that it promptly allows two different preparation means of the ceramic powders coated by a nanoparticle layer:    a) Synthesizing in the same (W/O) emulsion detonation step, both the ceramic powders intended to coat, as the nanoparticles that form the coating layer. For this purpose, two reaction kinetics are used differently for the precursors thereof, the formation reaction being extremely fast for the ceramic powders and slower in the case of decomposition of the precursors, which derive the nanoparticles that compose the coating layer.    b) Previously placing in the (W/O) emulsion starting composition the ceramic powders intended to coat, already prepared, together with the precursors that during detonation decompose into the nanoparticles that will form the coating layer.
Besides the said flexibility the process allows:    a. coating ceramic powders such as oxides, carbides, nitrides, inert metals, among others, from nanometric dimension to about 500 microns;    b. From the addition of different precursors to the (W/O) emulsion in stoichiometric proportions and of the combination among them during the detonation reaction, obtaining coatings with oxide nanoparticles in a multiplicity of crystalline structures (binary, ternary or superior), nitrides, inert metals, carbides, sulphides, etc;    c. As a result of the high temperatures, pressures and projection speed of the nanoparticles over the base particle, obtaining a coating with an extremely high adhesion grade;    d. Obtaining coatings with individual nanoparticles, without forming a continuous film, therefore allowing the change of the optical, mechanical, electrical, magnetic and catalytic properties of the base particles;    e. Obtaining crystallite sizes inferior to 50 and 20 nanometers both for the base particle and for the coating nanoparticles respectively;    f. High production capacities according to the high speed of the detonation reaction of the W/O emulsion.
The use of the (W/O) emulsion concept, for the nanoparticle synthesis, is referred in “Metal oxidize powder synthesis by the Combustion Method” Takoa Tami, Kazumasa Takatari, Vaovsashi Watanable and Nabuo Kaniya—Journal of Materials Research (1997). In this document a new method for the synthesis of nanometric alumina powders is disclosed (Emulaion Combustion Method—ECM), from the combustion with atmospheric air of an (W/O) emulsion, hollow alumina particles being obtained. However the method of the present invention uses a different regime, taken that the type of reaction occurring is detonation instead of combustion. Furthermore it does not require external air which allows a better control over the type of products and structures meant to be formed.
The document U.S. Pat. No. 5,255,827, in its turn, describes a cyclic process of detonation for the production of micrometric and nanometric powders and their projection at high-speed in different substrates, thus obtaining coated surfaces. The detonation happens in a gaseous mixture to which metals of fine granulometry are added, a suspension being formed. The process of the present invention stands out from the later due to the use of an (W/O) emulsion, in the liquid phase, to which solid precursors are added or dissolved, or still inert ceramic particles, making it possible to obtain individual particles coated with a nanoparticle layer.
More recently, references have been made to a (W/O) emulsion detonation for the production of ceramic materials, namely in a set of documents:
Document PT 103838, “Nanocrystalline spherical ceramic oxides, process for its synthesis and use thereof”, discloses a detonation method at high temperatures (superior to 2000° C.) of an (W/O) emulsion comprising metals in its composition and at least a dissolved precursor, for obtaining dense micrometrical spherical particles. In the coating method now disclosed, although also based on the (W/O) emulsion detonation, this emulsion is added with new types of precursors, the inert ceramic particles, intended to coat and solid nanoparticle precursors, that are critical elements for its final result, the ceramic powders coated by a nanoparticle layer.
The document PT 104085, “Ceramic materials of nanometric dimension, process for its synthesis and use thereof”, discloses a method for obtaining nanomaterials at low temperature (inferior to 2000° C.), from the detonation of two (W/O) emulsions, wherein the first is to stabilize the detonation front and the second has in its composition three types of precursors: dissolved oxidants in the internal phase, miscible fuels in the external phase and metals or alloys in the solid state, that combine during detonation in order to synthesize materials of nanometric dimension. However, in the method of the present invention, the (W/O) emulsion, additionally to the types of precursors already disclosed in the said document, comprises at least a precursor added to the (W/O) emulsion in the solid state, in way to guarantee sufficiently differentiated formation kinetics of the base particle and nanoparticles, in order to obtain a ceramic powder coated with nanoparticles. In another embodiment of the present invention, the inert ceramic particle intended to coat is directly added to the (W/O) emulsion and, unlike the precursors in the quoted document, it does not take part in the (W/O) emulsion detonation reaction, allowing the nanoparticles to deposit on its surface.
The document “Nano-MnFe2O4 powder synthesis by detonation of emulsion explosive” Xiao Hong Wang at al. discloses the synthesis of MnFe2O4 nanopowders from the detonation of an (W/O) emulsion, with the precursors of the nanoparticles dissolved in the (W/O) emulsion sensitized by a military explosive (RDX). However, in the process of the present invention, the (W/O) emulsion, besides the oxidants dissolved in its internal phase, equally referred in the sais document, requires at least a precursor added to the (W/O) emulsion in the solid state in way to guarantee sufficiently-differentiated formation kinetics of the base particle and of the nanoparticles, in order to obtain a ceramic powder coated by nanoparticles. In another embodiment of the present invention, the inert ceramic particle intended to coat is directly added to (W/O) emulsion and, unlike the precursors in the said document, it does not take part in the (W/O) emulsion detonation reaction, allowing the nanoparticles to deposit on its surface.